Browsing by Subject "Photoconductivity"

Now showing 1 - 10 of 10

Open Access
Chemically specific dynamic characterization of photovoltaic and photoconductivity effects of surface nanostructures
(American Chemical Society, 2010) Ekiz, O. Ö.; Mizrak, K.; Dâna, A.
We report characterization of photovoltaic and photoconductivity effects on nanostructured surfaces through light induced changes in the X-ray photoelectron spectra (XPS). The technique combines the chemical specificity of XPS and the power of surface photovoltage spectroscopy (SPV), with the addition of the ability to characterize photoconductivity under both static and dynamic optical excitation. A theoretical model that quantitatively describes the features of the observed spectra is presented. We demonstrate the applicability of the model on a multitude of sample systems, including homo- and heterojunction solar cells, CdS nanoparticles on metallic or semiconducting substrates, and carbon nanotube films on silicon substrates.
Open Access
Dynamical XPS measurements for probing photoinduced voltage changes
(2010) Sezen, H.; Süzer, Şefik
Photoillumination with 405 nm laser causes shifts in XPS peaks of n-Si(100), and CdS. To distinguish between surface photovoltage (SPV), and charging, dynamical measurements are performed, while sample is subjected to square wave pulses of ± 10.00 V amplitude, and 10-3-10 5 Hz frequency. For n-Si, Si2p peaks are twinned at + 10.00 and -10.00, yielding always 20.00 eV difference. Photoillumination shifts the twinned peaks to higher energies, but the difference is always 20.00 eV. However, for CdS, the measured binding difference of Cd3d peaks exhibits strong frequency dependence due to charging, which indicates that both fast SPV and slow charging effects are operative.
Open Access
Fermi level pinning ınduced by doping in air stable n type organic semiconductor
(American Chemical Society, 2020) Sharma, S.; Ghosh, S.; Ahmed, T.; Ray, S.; Islam, S.; Salzner, Ulrike; Ghosh, A.; Seki, S.; Patil, S.
Doping of organic semiconductors enhances the performance of optoelectronic devices. Although p-type doping is well studied and successfully deployed in optoelectronic devices, air stable ntype doping was still elusive. We succeeded with n-type doping of organic semiconductors using molecular dopant N-DMBI under ambient conditions. Strikingly, n-type doping accounts for a gigantic increase of the photoconductivity of doped thin films. Electrical and optical properties of the n-doped molecular semiconductor were investigated by temperature dependent conductivity, electron paramagnetic resonance (EPR), and flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements. A significant reduction and saturation in activation energy with increasing doping level clearly suggests the formation of an impurity band and enhancement in carrier density. Computational studies reveal the formation of a charge transfer complex mediated by hydrogen abstraction as the rate-determining step for the doping mechanism. The colossal enhancement of photoconductivity induced by n-doping is a significant step toward optoelectronic devices made of molecular semiconductors.
Open Access
High-performance solar-blind photodetectors based on AlxGa 1_xN heterostructures
(IEEE, 2004) Özbay, Ekmel; Bıyıklı, Necmi; Kimukin, I.; Kartaloglu, T.; Tut, T.; Aytür, O.
Design, fabrication, and characterization of high-performance AI xGa1-xN-based photodetectors for solar-blind applications are reported. AlxGa1-xN heterostructures were designed for Schottky. p-i-n, and metal-semicondnctor-metal (MSM) photodiodes. The solar-blind photodiode samples were fabricated using a microwave compatible fabrication process. The resulting devices exhibited extremely low dark currents. Below 3 fA, leakage currents at 6-V reverse bias were measured on p-i-n samples. The excellent current-voltage (I-V) characteristics led to a detectivity performance of 4.9×1014 cmHz1/2W -1. The MSM devices exhibited photoconductive gain, while Schottky and p-i-n samples displayed 0.09 and 0.11 A/W peak responsivity values at 267 and 261 nm, respectively. A visible rejection of 2×104 was achieved with Schottky samples. High-speed measurements at 267 nm resulted in fast pulse responses with greater than gigahertz bandwidths. The fastest devices were MSM photodiodes with a maximum 3-dB bandwidth of 5.4 GHz.
Open Access
Macroscopic photoconductive nanowire arrays
(IEEE, 2011) Bayındır, Mehmet; Yaman, Mecit; Özgür, Erol; Aktaş, Ozan; Khudiyev, Tural; Kanık, Mehmet; Deniz, Hakan
A novel high-throughput fabrication technique to produce polymer embedded functional chalcogenide nanowire arrays is demonstrated. Indefinitely-long selenium nanowire arrays are obtained and their size dependent photoconductivity is investigated. Logarithmic increase in photo-conductance is observed. © 2011 IEEE.
Open Access
Microwave whispering-gallery-mode photoconductivity measurement of recombination lifetime in silicon
(Advanced Electromagnetics, 2019) Yurchenko, V.; Navruz, T.; Çiydem, M.; Altıntaş, Ayhan
We present a whispering-gallery-mode resonance-enhanced microwave-detected photoconductivity-decay method for contactless measurement of recombination lifetime in highresistivity semiconductor layers. We applied the method to undoped Silicon wafers of high resistivity at 5 and 30 kOhm*cm and measured the conductivity relaxation times of 10 and 14 microseconds, respectively. In wafers being considered, they are supposed to be defined by the electron-hole diffusion from the bulk to the wafer surfaces.
Open Access
Photo-dynamic XPS for investigating photoinduced voltage changes in semiconducting materials
(2011) Sezen, Hikmet
The main motivation of this Ph.D. study is investigation of the photoinduced voltage changes in semiconductive materials with X-ray Photoelectron Spectroscopy (XPS). For this purpose, we have developed a technique for recording the shifts in the positions of the XPS peaks in response to different waveforms of electrical and/or optical stimuli for tracing dynamics of the developed potentials originating from intrinsic or extrinsic factors of the semiconductive materials such as charging/discharging, photoconductivity, surface photovoltage, band-bending/flattening/inversion, etc. Within this purpose, the surface photovoltage behaviors of n- and p-type doped Si and GaN samples are examined with the photo-dynamic XPS, to follow the behavior of the bandbending under photoillumination in both static and dynamic fashions. The band inversion effects are clearly observed on the n- and p-Si samples in the presence of a dielectric silica overlayer and on the p-GaN sample due to variation of the illuminating laser energies Moreover, the extent of the dopant dependent XPS peak shifts of the n- and p-Si samples are assessed after correction of their surface photovoltage values. A laser patterned silicon wafer with a high-power near infrared fiber laser is also investigated. While the patterned silica domains have identical chemical composition with the non-patterned regions, an investigation with dynamic XPS clearly reveals distinct dielectric characteristics of the patterned domains. Electrical parameters of CdS thin film are extracted by dynamic XPS with and without photoillumination. The photo-dynamic XPS technique has also provided useful information by disentanglement of processes; charging/discharging, photoconductivity, and surface photovoltage. Furthermore, location (space) dependent resistance and chemical profile of a CdS based Light Dependent Resistor (LDR) is also probed during realistic operational conditions, by utilizing spatially resolved XPS analysis (in the area mapping mode). In addition, with the XPS mapping analysis defects and malfunctioning sites/domains have been located under various experimental and preparation conditions.
Open Access
Photocurrent generation in a metallic transition-metal dichalcogenide
(American Physical Society, 2018) Mehmood, Naveed; Rasouli, Hamid Reza; Çakıroǧlu, Onur; Kasırga, T. Serkan
Photocurrent generation is unexpected in metallic 2D layered materials unless a photothermal mechanism is prevalent. Yet, typical high thermal conductivity and low absorption of the visible spectrum prevent photothermal current generation in metals. Here, we report photoresponse from two-terminal devices of mechanically exfoliated metallic 3R-NbS2 thin crystals using scanning photocurrent microscopy (SPCM) both at zero and finite bias. SPCM measurements reveal that the photocurrent predominantly emerges from metal/NbS2 junctions of the two-terminal device at zero bias. At finite biases, along with the photocurrent generated at metal/NbS2 junctions, now a negative photoresponse from all over the NbS2 crystal is evident. Among our results, we realized that the observed photocurrent can be explained by the local heating caused by the laser excitation. These findings show that NbS2 is among a few metallic materials in which photocurrent generation is possible.
Open Access
Probing dynamics of surface structures using x-ray photoelectron spectroscopy
(Simplex Academic, 2012) Sezen, H.; Süzer, Şefik
By incorporating laser illumination, and/or by application of electrical square-wave pulses to the sample, X-ray photoelectron spectroscopy is utilized to probe the dynamics of the surface structures in the range of 10-3 to 105 Hz. Different experimental methodologies are introduced, and their applications to investigate certain dynamical properties of two compound semiconductors, CdS and GaN are given. It is shown that dynamics of charging/discharging and surface photovoltage formation and its dissipation, and entanglement of various processes can be investigated by following the voltage shifts recorded in a chemical specific fashion.
Open Access
Trap levels in layered semiconductor Ga2SeS
(Elsevier, 2004) Aydınlı, Atilla; Gasanly, N. M.; Aytekin, S.
Trap levels in nominally undoped Ga2SeS layered crystals have been characterized by thermally stimulated current (TSC) measurements. During the measurements, current was allowed to flow along the c-axis of the crystals in the temperature range of 10-300 K. Two distinct TSC peaks were observed in the spectra, deconvolution of which yielded three peaks. The results are analyzed by curve fitting, peak shape and initial rise methods. They all seem to be in good agreement with each other. The activation energies of three trapping centers in Ga2SeS are found to be 72, 100 and 150 meV. The capture cross section of these traps are 6.7×10-23, 1.8×10-23 and 2.8×10-22cm2 with concentrations of 1.3×1012, 5.4×1012 and 4.2×1012cm-3, respectively.